1. Field of the Invention
The present invention relates to a fluorescent lamp having the optical output controlled.
2. Description of the Background
Japanese Unexamined Utility Model Publication No. 117558/1985 discloses a cold cathode fluorescent lamp having the optical output of a fluorescent lamp controlled without changing the emission wavelength by coating on the inner wall of a glass bulb, a phosphor containing a non-fluorescent substance, for example, a manganese-activated zinc orthosilicate phosphor powder or a cerium.terbium-activated lanthanum silicophosphate phosphor powder and a calcium pyrophosphate powder or an aluminum powder.
Further, U.S. Pat. No. 4,069,441 discloses a gas discharge lamp having at least two layers of phosphor particles laminated on the surface of a support, whereby a high level of optical output has been made possible by converting the radiant energy generated by gas discharge to a radiation with a long wavelength, by incorporating a non-fluorescent white substance which absorbs no more 20% of ultraviolet rays with a wavelength of 240 .mu.m or longer, for example, an ultraviolet reflective white substance such as barium sulfate or calcined calcium phosphate, to the phosphor layers except for the layer on the discharge side.
Furthermore, Japanese Unexamined Patent Publication No. 40854/1990 discloses a fluorescent lamp whereby high luminance and high color rendering are attained by coating a second layer comprising one or more phosphors on a first layer comprising a non-fluorescent substance of the formula LnPO.sub.4 (Ln=La, Gd, Y or Al), or by coating a mixture of such a non-fluorescent substance and phosphors.
Still further, European Patent No. 228010 discloses an inexpensive fluorescent composition with high luminance having a phosphor and an alkaline earth metal sulfate fused to each other.